The invention relates generally to the field of signal processing, and in particular to the field of channel source audio selection circuitry for an audio signal processor.
An audio signal processor is described in detail, for example, in a data sheet of Micronas Intermetall, “MSP 3410D Multistandard Sound Processor”, Edition Jan. 15, 1998, Order No. 6251-422-3PD. The block diagram shown on page 14 as FIGS. 4–5, “Audio Baseband Processing (DSP Firmware)”, includes a block labeled “Channel Source Select”, which, in conjunction with the associated circuit blocks, represents one example of such an audio source selection circuit.
In view of the great number of transmission systems for audio signals alone or in combination with television signals, the adaptation of the output channels to the respective type of signal in multistandard equipment is rather complicated. Besides the different audio standards, such as FM, AM, NICAM and other, particularly new standards in connection with digital television, the respective type of signal and the reproducer to be connected have to be taken into account. Both are classed in the following under the term “signal mode”. The type of signal indicates whether the audio signals to be reproduced are mono signals, stereo signals, or different language signals. The reproducers can be loudspeakers and headphones. In a broader sense, “reproducers” as used herein also means equipment connected via audio-video interfaces, for example via the well-known “SCART” socket. It is obvious that during direct reproduction via loudspeakers or headphones, A-language signals and B-language signals must never be output in the right-hand and left-hand sound channels simultaneously. On the other hand, it is advantageous if the loudspeakers reproduce the A-language signal, while the B-language signal can be heard over one or more headphones, or vice versa. If the signals are offered with sufficient quality, and stereo signals are present, switching to the latter should take place automatically. In the case of the audio-video interface, which does not directly lead to reproduction, it is advantageous to pass on or store the entire information contained in the audio signals, i.e., the A- and B-language contents, if present. In certain transmission standards, there is a third language channel, for example for linguistic minorities, which is to be reliably selectable as an output channel using an automatic audio source selection circuit.
In the prior-art audio source selection circuits, a source selection logic controlled by a controller is followed by an output matrix for each output channel. This output matrix is also controlled by the controller depending on the type of signal and on the reproducer connected to it. If an audio source provides only a mono signal, for example, the output matrix ensures that the mono signal is on both channel lines. If the audio source provides a stereo signal, the respective output matrix is switched to a transparent state, i.e., both signal components can pass through the output matrix. If the channel lines contain the A-language signal and the B-language signal, the respective output matrix may be switched to the transparent state only if a SCART socket is connected. In the other cases, both channel lines must be switched to either the A-language signal or the B-language signal.
From the foregoing it is apparent that, even if only few variants are taken into account, the setting or programming of the controller becomes very complicated. If the setting is performed in the factory or in a service shop, the effort required for the setting or programming is burdensome, but still feasible. Things are different if the owner of such a piece of multistandard equipment wants to alter the reproduction, for example to interchange the A- and B-language signals between the loudspeaker and headphone connections.